Sunday, September 27, 2009

Water-Demand makes Renewables Unsustainable.

It takes resources to get resources, especially those of water, one that is often overlooked in the various strategies of obtaining renewable energy. It is reckoned to take 2,500 gallons of water to grow sufficient corn to make one gallon of ethanol, against which the often quoted, but still sizeable, four gallons of water required to produce a gallon of ethanol from corn (or other source of sugar) is a mere drop in the ocean. Indeed, the intention to produce 36 billion gallons of ethanol per year by 2022, would use enough water to keep Chicago supplied for over 100 years. In Illinois, fields are adequately watered by rainfall whereas in more westerly and dryer regions it is necessary for farmers to actively irrigate their fields.

California has given a target of producing 1 million gallons of ethanol annually, but to grow enough corn to do so would need the entire volume of water that is currently diverted from the Sacramento-San Joaquin River Delta. Since this is the water that is used presently to irrigate 7 million acres of the Central Valley and provides water supplies for the cities of Southern California, it is debatable there is enough water to fulfil these purposes and that of wholesale ethanol production.

The problem of corn-ethanol, and its water demand, could be circumvented by making cellulosic ethanol instead but this technology is some years off from being one of large-scale production, and realistically, making ethanol in the quantities that are spoken of needs to get away from corn. In Brazil the sugar-cane ethanol industry is mature and is far less demanding in terms of water, since the crop is substantially supplied by rainwater.

I had not thought of there being a resource connection between solar-energy and water, but it seems there is. Photovoltaics are fairly independent of water, and generate clean (green) electricity with little demand once they are installed. The same is not true of solar power plants, for which a technology known as CST (Concentrating Solar Thermal) is more useful, in contrast to smaller, e.g. solar panels on the roof, type installations. CST employs an array of mirrors to focus sunlight onto a working fluid under pressure which is used to transfer heat to generate steam and then drive a steam-turbine to make electricity.

Solar thermal power plants (as do all power plants) produce waste heat, which is removed in cooling towers and released into the ambient atmosphere by the evaporation of water. In reality, beyond the initial stage that uses heat from the sun (and so is entirely renewable) the rest of the plant is that of any other kind of power plant and gets through huge quantities of water. Since it makes more sense to situate such CST installations is sunny spots (such as the desert Southwest), where there is year-round sunshine, there is an additional pressure therefore imposed on regions where securing adequate water supplies is already an issue, for example the Mojave Desert where it is planned to build 150 CST plants.

It is likely that heavy groundwater pumping would kill desert wildlife which depend on precious water from seeps and springs which would run dry. CST plants can be cooled by air but again the desert climate poses a problem, since higher outside temperatures decreases the efficiency of waste heat disposal, and wet-cooled plants are preferred because the heat-transfer is better, meaning they produce 5% more power and are 10% cheaper to build.

The problem is compounded by the effect of climate change which is blamed for a reduced flow in rivers out-west, and a consequent reduction in the amount of available hydroelectric power. The provision of water and energy are not independent agenda and need to be considered in a combined strategy.

Related Reading.
"When Renewable Is Not Sustainable," By Robert Glennon.

Friday, September 25, 2009

Night-Time Solar Energy.

Devices employing billions of heat collecting nanoantennas (“nantennas”) are under development, which may eventually provide a solar energy collector that is amenable to mass-production using flexible sheets, and will produce electricity at night. It is not presently possible to convert the energy collected to electricity but it is envisaged that once this hurdle is overcome, lightweight "skins" could be made to power all kinds of electrical devices from i-Pods to electric cars, at a higher efficiency than is possible with traditional PV cells. The nanoantennas also have the potential to cool buildings or electronics by collecting background infra-red (heat) energy which could be used to make electricity that could provide further cooling by powering air-conditioning units. Since they target mid-infrared rays, which the Earth continuously radiates as heat after absorbing energy from the sun during the day they could be used to produce electricity at night, in contrast with PV cells which are useless after dark. I.R.-driven PV cells are another route to providing night time solar electricity.

A nantenna is an electromagnetic collector designed to absorb specific wavelengths that are proportional to the size of the nantenna. Currently, Idaho National Laboratories has designed a nantenna to absorb wavelengths in the range of 3-15 μm. Since around 85% of the solar radiation spectrum contains light with shorter than infra-red wavelengths, in the range 0.4-1.6 μm it would be ideal to make nantennas of these dimensions to harvest more energy than is possible with PV. Nantennas work in practically the same way as rectifying antennas: namely that Incident light drags electrons in the antenna material back and forth at the same frequency as the incoming light, in consequence of the oscillating electric field component of the electromagnetic light wave. The refractive index of a material has a similar origin.

The oscillating electrons generate an alternating current (AC) in the antenna circuit, which must be rectified to convert it into DC power usually with a diode device of some kind, and the DC current can then be used to power an external load. Since the wavelengths in the solar spectrum lie in the approximate range 0.3-2.0 μm, a rectifying antenna needs to be on the order of hundreds of nm in size to provide an efficient energy collector. Since the oscillating (AC) frequency from the nantenna array is around 10 THz, converting it to the 50-60Hz power that the world uses poses a challenge in terms of using the technology to generate real usable power. The main problem with rectifying diodes is that they have a finite recovery time which limits their operating frequency. Commercially available ultrafast diodes presently have an upper limit of the order of several GHz, and so they need to be made to work faster. This seems to be the principal hurdle to the success of generating electricity using nantenna.

There have been many affirmations to the effect that the theoretical efficiency of nantennas is > 85%, which in comparison with the theoretical efficiency of single junction solar cells (30%) looks very impressive. There is some ambiguity over this, however, depending on exactly how the efficiencies are calculated for the two kinds of device.

The most obvious advantage of nantennas over semiconductor photovoltaics is that the nantenna arrays can be scaled to absorb any frequency of light. Since resonance frequency is in direct proportion to the size of the antenna, the array may be tuned by simply varying the size of the nantenna in the array to absorb specific light wavelengths. In the case of PV the frequency of absorbed light depends almost entirely on the band gap energy, and so the semiconductor material must be changed to vary the latter. Indeed, this aspect of dimensional engineering is in some ways reminiscent of nanotube and quantum dot devices. Although the latter work in quite different ways the point is made that it is not only the chemical composition of the material but the size of its assembly that provides a tuning to the absorption of light that is possible by a device.

Related Reading.

Tuesday, September 15, 2009

Algae Driven Batteries.

Taking the potential of algae into another dimension of energy production, research workers at Uppsala University have produced a novel lightweight battery by taking cellulose fibres from algae and coating them with a 50nm thin layer of polypyrrole.

The batteries have demonstrated charging capacities of between 25 and 33 mAh g−1 or 38−50 mAh g−1 per weight of the active material, can be charged with currents as high as 600 mA cm−2, and lose a mere six percent of their charging capacity after 100 charge/drain cycles. To quote from the link below, "In layman’s terms, these batteries are extremely light and can be charged in “11.3 seconds at 320 mA”.

The algae batteries have yet to be incorporated into a robust packaging which is another challenge for the team who have now made a battery that can take 1000 charges.

The batteries are of interest particularly because they should be cheap and amenable to mass production. However, as a consequence of their “low storage capabilities” they are unlikely to find application in e.g. MP3 players or laptops and certainly not in electric cars.

Prof. Maria Strømme said:

With the technique fully developed, I believe that we may see applications that we cannot really dream of today. Try to imagine what you can create when a battery can be integrated into wall papers, clothes, the packages of your medicines, etc.

At any rate it is interesting, as a cyborg device which does not require metals to make its essential working component, and even the polypyrrole conductor could be produced from biomass. That noted, I don't honestly see this as a saviour technology to obviate the energy crunch, but nor is it promised to be. Probably the most impacting use of algae in this respect is to make synthetic fuels, to replace increasingly scarce and costly oil and natural gas.

Related Reading.

"Green rechargable batteries are made from algae."

Monday, September 14, 2009

Russian Oil Production Peaks.

Since the year 2000, Russian oil production has increased by practically 50%, but this growth appears to have now peaked. The supply on Non-OPEC oil peaked early in this decade and it was only Russia, returning to force from the prior financial crisis that could offset the fall in the remaining parts of this sector. Non-OPEC accounts for about 60% of world oil production, but within the sector it is Russia alone that has maintained the plateau, providing almost one quarter of its output. Without Non-OPEC it will not be possible to raise world oil supply, and without Russia it would have already fallen. Russia alone could not maintain growth in Non-OPEC, and the peak in Russian supply means that it and world oil volumes must begin to decline.

The unavoidable fall in world oil production has excited the potential for exploration in extremely inhospitable regions of the world, particularly the Arctic. As a kind of dry-run for exploration above the Arctic circle, the Nordic Explorer vessel has "sailed" for Cape Farewell on the southern tip of Greenland. I cannot avoid thinking that the term "Farewell" is an ironic coincidence for the future of a world powered by oil, and the desperation to grab whatever of it is left to be grabbed; wherever that may be. According to the US Geological Survey, there could be as much as 50 billion barrels worth of oil under Greenland, which is around eighteen months worth for the word as a whole and can be compared with the 38 billion barrels produced in the North Sea since development of the region began in the 1960s.

Exploration of Greenland is not new, but so far the few wells that have been drilled there proved to be dry. However, with an inevitable long-term rising price of increasingly scarce oil and rising demand for it, further exploration projects there begin to look viable, on the basis that sooner or later someone will strike lucky. Global warming may prove an ally in this intention, since hitherto ice-blocked waterways will become open, thus rendering greater access to whatever oil and other mineral wealth may lie there. In the past two years, seven companies including Exxon Mobil, Chevron and the UK-based Cairn Energy have bought exploration blocks of southern and western Greenland.

In consequence of the long-term production of North Sea oil, the reserves there are notably depleted and it will require considerable investment and new technologies to get out what remains. The low price of a barrel of oil in consequence of last year's economic crash has discouraged many putative exploration projects, and now the US based Noble Energy has put its North Sea business on the market for $350 million. The firm thus joins an exodus of UK based oil-companies from the region in a move where long-established fields are sold-off in order to fund exploration in new regions, including deepwater projects and indeed the Arctic.

Related Reading.
[1] "Oil Supply: As Russian Production Tops Out, World Supply Will Continue to Slip," By Gregor Macdonald.
[2] "Oil giants zero in on untapped Greenland."
[3] "American oil group Noble Energy joins UK exodus from North Sea," By Danny Fortson.